The invention relates to a method for navigating a vehicle, in which an actual position of the vehicle is ascertained using the length and direction of distances covered, a control position is ascertained at intervals of time using external auxiliary means, and the actual position is reset to the control position if the gap between the actual position and the control position exceeds an error value.
Such a method is used in a xe2x80x9cCarinxe2x80x9d navigation system, for example.
In the context of vehicle navigation, a distinction is drawn between three subtasks, namely, first position determination, second, route planning, and third, transmitting the route to the driver. The present invention relates to the first part, namely determining the position of the vehicle. It also extends to route planning and to transmission of the route found to the driver.
To determine the position of a vehicle, navigation satellites forming part of a global positioning system or GPS have been available for some time. These satellites radiate a signal. The transit time of this signal can be used to determine the distance from each individual satellite. If the distance from at least three satellites is known, then it is possible to ascertain the position on the earth. However, GPS position determination is generally subject to an error in the order of magnitude of 50 to 100 m. This error can be tolerated while driving on an inter-urban journey. It poses problems, however, when the distance between roads is less than this tolerance threshold, for example in towns or at traffic junctions.
Another type of navigation involves adding the distances covered with their directions on a vector basis, starting from a starting point. This procedure is also called compound navigation or xe2x80x9cdead reckoningxe2x80x9d. Such navigation also produces errors which, by way of example, are based on the fact that the distance covered in one direction and the direction itself cannot be determined with sufficient accuracy. If, by way of example, the distance is ascertained with an error of 1%, then, after a distance of 10 km, the errors which also arise for GPS navigation are again produced in the order of magnitude of 100 m.
Hence, in the case of the xe2x80x9cCarinxe2x80x9d system mentioned in the introduction, the two navigation methods have been coupled. In principle, the actual position of the vehicle is ascertained using compound navigation, i.e. using sensors which are attached to the vehicle and ascertain the direction in which the vehicle is moving and the distance covered in this direction. From time to time, however, the calculated actual position is checked using external auxiliary means, for example the GPS satellites. If the two positions match, then the actual position continues to be regarded as reliable. If the gap between the actual position and the control position exceeds a predetermined fixed error value, then the control position is used as the new starting point for further navigation.
The invention is based on the object of improving position determination.
This object is achieved for a method of the type mentioned in the introduction by virtue of the error value being changed on the basis of a reliability check.
Thus, a fixed error value is no longer prescribed as previously, for example a gap of 250 m, with the actual position being reset to the control position if said gap is exceeded, but instead this error value or gap is designed to be variable. This can mean that the gap becomes greater than previously under certain circumstances. In this case, a correction is made less frequently or later. However, it can also mean that the error value becomes smaller than previously, so that corrections need to be made more frequently or earlier. The reliability can also incorporate the xe2x80x9cpastxe2x80x9d, for example. If it has been found in the past that the actual positions ascertained using compound navigation are reliable, then the actual positions can be trusted more and a correction can be made less often.
It is also advantageous if the reliability is determined using at least one comparison position. The comparison positions can be used to estimate whether the actual positions have been ascertained correctly.
In this case, it is preferable for the comparison position to be determined using external auxiliary means. The external auxiliary means are independent of the calculation of the position using compound navigation. This produces an objective criterion which contributes to improved assessment of the reliability, even if the comparison position is likewise erroneous.
In this case, a relatively high degree of reliability is assumed if the profile of the actual positions matches the profile of the comparison positions, or the gap between the actual positions and the comparison positions is small over a plurality of determination operations. In this case, small means a range of below 100 m, for example. Thus, if the actual positions follow the profile of the comparison positions, then it can be assumed that compound navigation has worked reliably and the result is correct. In this case, it is possible to permit a greater error or gap before returning to the control position.
In this context, a particular preference is that a larger error value is assumed for a high degree of reliability than for a low degree of reliability. If, on the other hand, the degree of reliability was lower in the past, then a smaller error value will be used in order to be able to make a correction early on.
Preferably, the reliability value is changed whenever a comparison position has been determined, so long as the reliability value is situated within a predetermined value range. If, for example, it has been found that the actual position and the comparison position have been situated close enough together, then the reliability value can be enlarged, for example by a fixed increment. The reliability value used can be a distance value which may possibly also match the error value. The next time the comparison position is determined, the control position and the actual position could already be situated somewhat further apart without this necessarily resulting in a correction. So long as the gap between the actual position and the comparison position is smaller than the reliability value, the result of this is that the reliability value is enlarged again. If, on the other hand, it is found that the gap is larger than the enlarged reliability value, then this again results in the reliability value being reduced. Accordingly, this also applies to other criteria used to determine the reliability, for example the match between the profiles. However, the reliability value will neither be allowed to become infinitely large nor will it be set to negative numbers. It will always be left in the range from 0 to 50,000 cm, for example. Thus, if the reliability value has reached the upper limit, the reliability value will not be enlarged after the comparison position has been determined.
Preferably, the reliability values are subdivided into classes, and each class is assigned a predetermined error value. The result of this is a restriction to a predetermined number of error values, that is to say, for example, gaps staggered at 60 m, 120 m, 180 m, 240 m and 300 m. It is thus not necessary to convert the reliability value to an error value, but rather it can be found in a table. This simplifies management.
Preferably, the distance covered between two positions is used to change the reliability value. This is an alternative embodiment to the possibility of enlarging the reliability value by fixed increments. The advantage of this procedure is that the distance covered has an influence on the reliability check. If, after a relatively long distance covered, there is still a sufficient match between the actual position and the comparison position, then the reliability value can be increased to a greater extent than if the match exists only after a short distance. The size of the distance covered depends on the frequency at which the comparison positions are determined. The aim is to determine the comparison position at a frequency of 5 Hz. Depending on the speed, the vehicle covers between approximately 1 and approximately 10 m in this case.
Preferably, the external auxiliary means used are satellites. The satellites are available virtually everywhere. They are independent of the vehicle and can therefore be used relatively objectively to determine the comparison position.
As an alternative or in addition to this, the external auxiliary means used can be roads on which the vehicle is depicted. These are virtual roads but largely correspond to the actual roads. In this case, it is assumed that a vehicle will, with a high degree of probability, be moving on a road if such a road is close by. If, therefore, compound navigation has been used to calculate a position which is not all that far away from a position on a road, then the road position can be used as the comparison position. Depiction on the road is also called map matching.
The roads are preferably stored in a database. Thus, depiction on the road is purely numerical.
Preferably, the reliability value is reduced if valid depiction of the vehicle on a road is not possible. Thus, an additional attempt to depict the vehicle on the road, that is to say to perform map matching, is made each time a control position is determined, or more frequently. So long as this is possible, the system assumes that the vehicle is situated on the road. If the system does not find any corresponding road close by on which the vehicle can be depicted, then it changes from an xe2x80x9con roadxe2x80x9d mode to an xe2x80x9coff roadxe2x80x9d mode. This change is an indication that position determination is becoming less well defined or more inaccurate. The reliability is somewhat reduced.
A further preferred refinement is that the reliability value is reduced if the gap between successive comparison positions is situated outside of a range whose limits are determined by predetermined multiples of the gap between the corresponding actual positions. If the gap between the actual positions is, for example, 0.5 to 2 times the gap between the comparison positions, it is assumed that position determination using compound navigation has been carried out with the required degree of reliability. If the gap between the actual positions is situated outside of this range, that is to say it is smaller than 0.5 times or larger than 2 times the gap between the comparison positions in this case, then it is assumed that an error has occurred and the reliability value is reduced, that is to say the monitoring is intensified.
It is also advantageous if the control positions are filtered. Abrupt changes when control positions are ascertained then have less of an influence on determination of the control positions.